A common mode of failure for cutting inserts is cracking due to thermal shock. Thermal shock is even more common in the more difficult machining processes, such as high productivity machining processes and machining of materials with a high hot hardness, for example. In order to reduce the buildup of heat in cutting inserts, coolants are used in machining operations. However, the use of coolants during the machining operation contributes to thermal cycling that may also contribute to failure of the cutting insert by thermal shock.
Thermal cycling also occurs in milling applications where the milling cutter gets hot when actually cutting the work material and then cools when not cutting the work material. Such thermal cycling of heating and cooling results in sharp temperature gradients in the cutting inserts, and the resulting in differences in expansion of different portions of the insert causing internal stresses and initiation of cracks in the cutting inserts. There is a need to develop a novel carbide cutting insert that can not only maintain efficient cutting performance during the high-hot hardness machining process, but also improve the tool life by resisting thermal cracking.
The service life of a cutting insert or cutting tool is also a function of the wear properties of the cemented carbide. One way to increase cutting tool life is to employ cutting inserts made of materials with improved combinations of strength, toughness, and abrasion/erosion resistance. Cutting inserts comprising cemented carbide substrates for such applications is predicated on the fact that cemented carbides offer very attractive combinations of strength, fracture toughness, and wear resistance (such properties that are extremely important to the efficient functioning of the boring or drilling bit). Cemented carbides are metal-matrix composites comprising carbides of one or more of the transition metals as the hard particles or dispersed phase and cobalt, nickel, or iron (or alloys of these metals) as the binder or continuous phase. Among the different possible hard particle-binder combinations, cemented carbides comprising tungsten carbide (WC) as the hard particle and cobalt as the binder phase are the most commonly used for cutting tools and inserts for machining operations.
The bulk properties of cemented carbides depend upon, among other features, two microstructural parameters, namely, the average hard particle grain size and the weight or volume fraction of the hard particles and/or the binder. In general, the hardness and wear resistance increases as the grain size decreases and/or the binder content decreases. On the other hand, fracture toughness increases as the grain size increases and/or as the binder content increases. Thus there is a trade-off between wear resistance and fracture toughness when selecting a cemented carbide grade for any application. As wear resistance increases, fracture toughness typically decreases and vice versa.
In addition, alloying agents may be added to the binder. A limited number of cemented carbide cutting tools or cutting inserts have ruthenium added to the binder. The binder may additionally comprise other alloying compounds, such as TiC and TaC/NbC, to refine the properties of the substrate for particular applications.
Ruthenium (Ru) is a member of the platinum group and is a hard, lustrous, white metal that has a melting point of approximately 2,500° C. Ruthenium does not tarnish at room temperatures, and may be used as an effective hardener, creating alloys that are extremely wear resistant. It has been found that ruthenium in a cobalt binder of a cemented carbide used in a cutting tool or cutting insert improves the resistance to thermal cracking and significantly reduces crack propagation along the edges and into the body of the cutting tool or cutting insert. Typically commercially available cutting tools and cutting inserts may include a concentration of ruthenium in the binder phase of cemented carbide substrates in the ranges of approximately 3% to 30%, by weight.
A cutting insert comprising a cemented carbide substrate may comprise a single or multiple layer coating on the surface to enhance its cutting performance. Methods for coating cemented carbide cutting tools include chemical vapor deposition (CVD), physical vapor deposition (PVD) and diamond coating. Most often, CVD is used to apply the coating to cutting inserts due to the well-known advantages of CVD coatings in cutting tools.
An example of PVD coating technologies, Leyendecker et al. discloses, in a U.S. Pat. No. 6,352,627, a PVD coating method and device, which is based on magnetron sputter-coating techniques to produce refractory thin films or coats on cutting inserts, can deliver three consecutive voltage supplies during the coating operation, promoting an optimally enhanced tonization process that results in good coating adhesion on the substrate, even if the substrate surface provided is rough, for example because the surface was sintered, ground or jet abrasion treated.
An example of CVD coating technologies, Punola et al. discloses, in a U.S. Pat. No. 5,462,013, a CVD coating apparatus that uses a unique technique to control the reactivity of a gaseous reactant stream at different coating zones in the CVD reactor. As a result, the CVD coating produced has greatly improved uniformity in both composition and thickness.
An example of hard-metal coating developments and applications in cutting inserts with regular carbide substrates, Leverenz and Bost from Stellram, an Allegheny Technologies Company located at One Teledyne Place, LaVergne, Tenn., USA 37086 and also the assignee of this invention, describes in a recently granted U.S. Pat. No. 6,929,851, a surface etching technology that is used to enhance the CVD or PVD coating including HfCN coating on the regular carbide substrates. Additional examples of hard-metal coating developments and applications in cutting inserts with regular carbide substrates are U.S. Pat. No. 4,268,569 by Hale in 1981, U.S. Pat. No. 6,447,890 by Leverenz et al. in 2002, U.S. Pat. No. 6,617,058 by Schier in 2003, U.S. Pat. No. 6,827,975 by Leverenz et al. in 2004 and U.S. Pat. No. 6,884,496 by Westphal and Scottke in 2005.